a. Field of the Invention
This invention relates to a thermal head having resistive heater elements composed of a metal boride as the principal constituent, and to a method for producing the same.
b. Description of the Prior Art
A thermal head for use in thermal print-recording is so constructed that a plurality of resistive heater elements and electrically conductive bodies to supply electric power to these resistive heater elements are provided on a substrate, or base plate, such as glass having the electric insulating property and a flat and smooth surface.
This type of thermal head is operated in such a manner that, in order to obtain a required heat pattern in accordance with information to be recorded, electric current is caused to flow through the resistive heater elements via corresponding electrically conductive bodies to thereby generate heat, and that the thus heat-generated head is contacted to a recording medium to effect recording of the information.
For the resistive heater element to be used in the thermal head, there have heretofore been known a thin film resistive heater element made of tantalum nitride, nickel-chromium alloy, etc., a thick film resistive heater element using silver-palladium alloy, etc., and a semiconductor resistive heater element using a silicon semiconductor. Of these, the thin film resistive heater element possesses such characteristics that it has higher heat-response, is more excellent in the heat-resistance and thermal shock resistance, and has a longer service life and higher reliability than those of the thick film resistive heater element and the semiconductor resistive heater element.
For this thin film resistive heater element, tantalum nitride has been particularly used heretofore, since the compound has a relatively high heat-resistant property, high reliability, and a comparatively high resistivity of 250 to 300 .mu..OMEGA.cm, providing satisfactory controlling capability in its fabrication. This resistive heater element made of tantalum nitride is disclosed, for example, in U.S. Pat. No. 3,973,106.
However, tantalum nitride has such disadvantages that it is rapidly oxidized at a high temperature of approximately 300.degree. C. and above to abruptly increase its resistance value, the increase of which deteriorates density in printing when the output informations are printed on a recording paper. In order to avoid such disadvantages, it has so far been a general practice in the manufacture of the thermal head using the tantalum nitride resistive heater element to provide an oxidation-resistant protective layer of silicon oxide (SiO.sub.2) on the tantalum nitride resistive heater element and to provide over this protective layer a wear-resistant layer of tantalum oxide (Ta.sub.2 O.sub.5). This combined protective layer of silicon oxide and tantalum oxide is disclosed in U.S. Pat. No. 3,931,492. Even with the thermal head utilizing this protective layer and the abovementioned tantalum nitride resistive heater element in combination, variations in resistance of such thermal head, when it is driven for a long period of time, is small and still unsatisfactory. In view of recent trend of increasing demand to thermal head for high speed recording, it is necessary for a narrow the pulse width for electric conduction through the thermal head so as to form color on heat-sensitive paper. In this consequence, the power consumption will increase more than ever, and the resistive heater element will increase its temperature to a higher level with the consequence that the thermal head shortens its service life. On account of this, there has been a demand for the resistive heater element having much higher heat resistance. The sheet resistance of tantalum nitride is usually 50 ohm/.quadrature. or so, and, even when the area is particularly enlarged for the thermal head, it is 100 ohm/.quadrature. or so. In order to further increase the resistance value, there are employed various expedients such as trimming, thinning of the film thickness, and so forth, although disadvantages such as complexity in its manufacturing processes, mal-effect to the service life, and so on would inevitably arise. Thus, with the thin film resistive heater element of tantalum nitride, it is not possible to take a large sheet resistance with the consequence that current flow should inevitably increase to feed the electric power necessary for heating the element, hence the resistance value of the electrically conductive body to be used for the electrode and circuit wiring becomes a problem. In other words, since the resistance value of the electrically conductive body with respect to the resistance value of the thin film resistive heater element becomes innegligible, quantity of heat-generation in each resistive heater element becomes different depending on the difference in the distance of each electrically conductive body connected to the resistive heater element, whereby there occurs a difference in density of the recorded pattern, and the quality of the recorded information becomes poor. When the size of the thin film resistive heater element is reduced to further increase the recording density, the power consumption in the electrically conductive body becomes a problem, since the resistance value of the electrically conductive body alone increases, while the sheet resistance of the thin film resistive heater element remains constant. On the other hand, when thickness of the electrically conductive body is extremely increased to avoid such problem of power consumption, the surface irregularity of the electrically conductive body becomes prohibitive at the time of multi-layer wiring to bring about serious structural inconveniences such as decreased durability against wear and tear of the conductive body. Furthermore, large current flow would inevitably lead to increased capacity of the heating power source, switching circuit, and so forth.
The substrate constituting the conventional thermal head still has a room for further improvement in that its heat responsive characteristic should be improved for increasing its recording speed. In more detail, temperature of the heat-generating section in the thermal head generally varies as shown in FIG. 1. In the graphical representation shown in FIG. 1, t.sub.1 denotes a time, during which electrical pulses are applied to the heat generating section, and t.sub.2 indicates a time, during which the thermal head returns to its initial temperature starting from the time instant when application of the electrical pulses is interrupted.
In order to increase the thermal recording speed, it is effective to shorten the time t.sub.2 so that subsequent input of the electrical pulses may be done quickly. If the subsequent pulses are applied to the heat-generating section of the thermal head, while its state of cooling is incomplete, and the pulse application is repeated sequentially, the thermal head is brought to a temperature higher than a color-forming limit temperature T.sub.1 as shown in FIG. 2 due to heat accumulating effect with the result that unnecessary recording is effected.
The substrate for the thermal head is desired to have the following properties.
(1) The surface thereof should be perfectly flat and smooth.
(2) The substrate should be able to efficiently transfer heat generated in the resistive heater element to heat-sensitive paper, and be small in its temperature increase due to heat accumulation.
(3) The substrate should be free from warping and twisting, and have satisfactory flatness (so that the photoetching technique may be applied thereto when fabricating delicate patterns on it such as the resistive heater element, electrodes, and so on).
(4) The substrate should be less in alkali ions which are liable to cause deterioration of the resistive heater element.
To meet these various conditions as mentioned above, glazed ceramics which consists of alumina ceramics coated thereon with a glass layer has generally been used very much.
Further, the glass layer for this glazed ceramics is required to have the following properties.
(1) It should have a thermal expansion coefficient similar to that of alumina (to maintain the required flatness).
(2) It should have good adhesion to alumina.
(3) It should be able to produce a flat and smooth glazed surface.
At the present stage, the glazed layer of the glazed ceramics having a high softening temperature to satisfy these various conditions is difficult to obtain, hence its maximum operating temperature is limited.
On the other hand, in view of increasing demand to the high speed thermal recording operation, the resistive heater element for the thermal head is required to have more durability against high temperature than ever.
As the results of various studies and experiments, therefore, it has been found out that, for improving durability of the thermal head at a high temperature, the substrate, on which the resistive heater element is to be formed, should be improved along with improvement in the resistive heater element per se.
Furthermore, for the electrically conductive body to be provided on the thermal head, there have been used various electrically conductive material such as gold, silver, copper, aluminum, etc. and their alloys having a low resistivity with respect to a certain definite film thickness, and being stable both chemically and thermally. Of these electrically conductive bodies, copper, aluminum, and their alloys have generally strong bonding force to the substrate. However, silver and gold have poor bonding force and therefore, in order to improve their adhesivity to the substrate, it has so far been the practice to coat a thin film of chromium or nickel-chromium alloy on the substrate as the priming layer, although even such priming layer has been found not sufficiently effective to the resistive heater element composed of the metal boride as the principal constituent.